[invoker] Use a DBus method to notify lipstick an app is started

This patch adds a new -F/--desktop-file command line option to
invoker which takes a desktop file name as argument. Using it will
notify lipstick that  the app which goes with that desktop file is
being started.


This needs https://github.com/nemomobile/lipstick/pull/222
From the docs, it sounds like you'd need a `dbus_connection_flush` for this to work.